In the field of semiconductor processing, a number of techniques have been described to convert thin amorphous silicon films into polycrystalline films. One such technique is sequential lateral solidification (“SLS”). SLS is a pulsed-laser crystallization process that can produce polycrystalline films having elongated crystal grains on substrates, such as, but not limited to, substrates that are intolerant to heat (e.g., glass and plastics). Examples of SLS systems and processes are described in commonly-owned U.S. Pat. Nos. 6,322,625, 6,368,945, 6,555,449, and 6,573,531, the entire contents of which are incorporated herein by reference.
SLS uses location controlled laser pulses to melt a region of an amorphous or polycrystalline thin film on a substrate. The melted regions of film then laterally crystallize into a directionally solidified microstructure or a multitude of location-controlled large single-crystal regions. Generally, the melt/crystallization process is sequentially repeated over the surface of a thin film. One or more devices, such as image sensors, active-matrix liquid crystal displays (“AMLCD”), and active-matrix organic light-emitting diode (AMOLED) display devices, can then be fabricated from the crystallized film. In the AMLCD and AMOLED display devices, a regular array of thin-film transistors (“TFTs”) or TFT circuits is fabricated on a transparent substrate, and each transistor or circuit serves as a pixel controller.
In conventional SLS systems, one factor in successful crystallization is the precision of the stages that translate the sample with respect to the laser pulses. For current Gen-4 two dimensional (“2D”)-projection SLS systems, translation velocities of the stages are on the order of tens of cm/s, for example, 18 cm/s. Stages such as these have certain deviations from a perfectly straight line of motion. That deviation will be collectively referred to herein as stage wobble. As used herein, “stage wobble” refers to variations and deviations of the stage position from its intended position as it translates in the laser path. Such variations can be, for example, when the stage is moving in the x-direction, unintended small motion of the stage in the y-direction. A 2D projection system creates a two dimensionally patterned beam for performing SLS. Other methods can create line beams for performing SLS.
One issue related to stage wobble in conventional single-scan two-shot SLS is the non-equidistant spacing of long grain boundaries in material made from two sequential laser pulses, i.e., a two-shot material. A single scan SLS process refers to an SLS process that can fully crystallize a region on a substrate in a single scan. Two-shot SLS refers to a SLS process that fully crystallizes a given portion of such a region with two laser pulses. The wobble of the stages between two pulses can result in a non symmetric overlapping of the second pulse with the first pulse. Ideally, beamlets of the second pulse are centered between regions irradiated by beamlets of the first pulse so as to achieve a constant spacing between the grain boundaries created by the two-shot process. If the beamlets of the second pulse are not well positioned because of stage wobble, the grains in one column can be shorter than in the grains in a neighboring column and many grains can remain in the wider column that are not fully extending the width of the column (e.g., occluded grains). Furthermore, beamlet distortion, caused by various aberrations in the projection optics, also may result in locally non symmetric overlapping of the second pulse in the scan. As used herein “beam distortion” refers to aberrations in the projection optics that can result in non-uniform beamlet formation.